Vertically separated acoustic filters and resonators

ABSTRACT

An apparatus including vertically separated acoustic resonators are disclosed. The apparatus includes a first acoustic resonator on a substrate and a second acoustic resonator vertically separated above the first acoustic resonator. Because the resonators are vertically separated above another, total area required to implement the resonators is reduced thereby savings in die size and cost are realized. The vertically separated resonators are supported by standoffs that are fabricated on the substrate, or on a resonator.

CROSS-REFERENCE TO RELATED APPLICATION

This continuation application claims the benefit under 35 U.S.C. Section120 of U.S. patent application Ser. No. 10/785,525, entitled “VerticallySeparated Acoustic Filters And Resonators” by Richard C. Ruby and JohnD. Larson, III filed Feb. 23, 2004 now U.S. Pat. No. 7,038,559 which isincorporated herein by reference.

BACKGROUND

The present invention relates to acoustic resonators, and moreparticularly, to resonators that may be used as filters for electroniccircuits.

The need to reduce the cost and size of electronic equipment has led toa continuing need for ever smaller filter elements. Consumer electronicssuch as cellular telephones and miniature radios place severelimitations on both the size and cost of the components containedtherein. Many such devices utilize filters that must be tuned to precisefrequencies. Hence, there has been a continuing effort to provideinexpensive, compact filter units.

One class of filters that has the potential for meeting these needs isconstructed from thin film bulk acoustic resonators (FBARS). Thesedevices use bulk longitudinal acoustic waves in thin film piezoelectric(PZ) material. In one simple configuration, a layer of PZ material issandwiched between two metal electrodes.

The sandwich structure is preferably suspended in air by a supportstructure. When electric field is applied between the metal electrodes,the PZ material converts some of the electrical energy into mechanicalenergy in the form of mechanical waves. The mechanical waves propagatein the same direction as the electric field and reflect off of theelectrode/air interface.

At a resonant frequency, the device appears to be an electronicresonator. When two or more resonators (with different resonantfrequencies) are electrically connected together, this ensemble acts asa filter. The resonant frequency is the frequency for which the halfwavelength of the mechanical waves propagating in the device is equal tothe total thickness of the device for a given phase velocity of themechanical wave in the material. Since the velocity of the mechanicalwave is four orders of magnitude smaller than the velocity of light, theresulting resonator can be quite compact.

In designing and building miniature filters for microwave frequencyusage, it is often necessary to provide multiple interconnectedresonators (for example, FBARS) fabricated on a die. FIG. 1 is aschematic diagram showing a portion 10 of a filter circuit. Forconvenience, the illustrated portion is referred to herein as the“filter circuit” 10. The filter circuit 10 includes a plurality ofinterconnected resonators. Referring to FIG. 1, some of the illustratedresonators are connected in series and are referred to as seriesresonators 12, 14, and 16 while other illustrated resonators areconnected in parallel and are referred to as shunt resonators 22, 24,26, and 28. The filter circuit 10 connects to external circuits (notillustrated) via connection points 11, 13, 15, 17, 19, and 21.

FIG. 2 shows a top view of a die 20 illustrating topology of theresonators of the filter circuit 10 FIG. 1 as they are typicallyimplemented on the die 20. In FIGS. 1 and 2, corresponding resonatorsare illustrated with same reference numerals. Connection points of FIG.1 are illustrated as connection pads in FIG. 2 and correspondingconnection points and connection pads are illustrated with samereference numerals.

As illustrated, the die 20 requires a die area (defined by the first andsecond dimensional extents illustrated as X-axis extent 23 and Y-axisextent 25) to implement the resonators. Die area is a scarce andexpensive resource in many electronic devices, for example, wirelesscommunication devices such as cellular telephones. It is desirable to beable to implement the filter circuit 10 on a smaller die allowing formanufacture of smaller and less costly devices.

SUMMARY

The need is met by the present invention. In a first embodiment of thepresent invention, an apparatus includes a first acoustic resonator on asubstrate and a second acoustic resonator vertically separated above thefirst acoustic resonator such that little or no acoustic energy iscoupled between the first acoustic resonator and the second acousticresonator. Because the resonators are vertically separated, they requireless die space resulting in smaller and more area efficient and costeffective implementation of the apparatus.

In a second embodiment of the present invention, an apparatus includes aplurality of resonators fabricated on a substrate where a first acousticresonator is fabricated on the substrate and a second acoustic resonatoris vertically separated and acoustically separated above the firstacoustic resonator.

In a third embodiment of the present invention, a method of fabricatingan apparatus is disclosed. First, a first resonator is fabricated on asubstrate. Then, a sacrificial layer is fabricated surrounding the firstresonator. Standoffs are fabricated. Next, a second resonator isfabricated on the sacrificial layer above the standoffs. Finally, allthe sacrificial layer are removed.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a portion of a filter circuitincluding a plurality of resonators;

FIG. 2 is a top view of a die implementing the filter circuit of FIG. 1using prior art topology;

FIG. 3 is a schematic diagram of the filter circuit of FIG. 1 redrawn toemphasize topology of resonators in accordance with one embodiment ofthe present invention;

FIG. 4 is a top view of a die implementing the filter circuit of FIG. 3in accordance with one embodiment of the present invention;

FIG. 5 is a cutaway side view of a portion of the die of FIG. 4 cutalong line A—A;

FIG. 6 is a cutaway side view of the portion of the die of FIG. 4 cutalong line A—A during its fabrication process;

FIG. 7 is a schematic diagram of the filter circuit of FIG. 1 redrawnagain to emphasize topology of resonators in accordance with anotherembodiment of the present invention;

FIG. 8 is a cutaway side view of a die implementing the filter circuitof FIG. 7.

DETAILED DESCRIPTION

The present invention will now be described with reference to the FIGS.1 through 8 which illustrate various embodiments of the presentinvention. In the Figures, some sizes of structures or portions may beexaggerated relative to sizes of other structures or portions forillustrative purposes and, thus, are provided to illustrate the generalstructures of the present invention. Furthermore, various aspects of thepresent invention are described with reference to a structure or aportion positioned “above” or “right of” relative to other structures,portions, or both. As will be appreciated by those of skill in the art,relative terms and phrases such as “above” or “right of” are used hereinto describe one structure's or portion's relationship to anotherstructure or portion as illustrated in the Figures. It will beunderstood that such relative terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe Figures. For example, if the device in the Figures is turned over,rotated, or both, the structure or the portion described as “above” or“right of” other structures or portions would now be oriented “below” or“left of” the other structures or portions.

As shown in the figures for the purposes of illustration, embodiments ofthe present invention are exemplified by an apparatus having a firstacoustic resonator on a substrate and a second acoustic resonatorvertically separated above the first acoustic resonator. Because theresonators are vertically separated above another, total area requiredto implement the resonators is reduced thereby savings in die size andmanufacturing costs are realized.

FIG. 3 is a schematic diagram of the filter circuit 10 of FIG. 1 redrawnto emphasize topology of resonators in accordance with one embodiment ofthe present invention. The redrawn filter circuit is designated usingreference numeral 10 a. The filter circuit 10 a of FIG. 3 includesidentical component resonators and connection points as the filtercircuit 10 of FIG. 1. Further, operations of the filter circuit 10 a ofFIG. 3 are identical to operations of the filter circuit 10 of FIG. 1.For this reason, same reference numerals are used for the correspondingcomponents in FIGS. 1 and 3 except as follows: in FIG. 3, to moreclearly illustrate the vertically separating technique in accordancewith the illustrated embodiment of the present invention, seriesresonators 12, 14, and 16 of FIG. 1 are referred to as series resonators12 a, 14 a, and 16 a in FIG. 3.

FIG. 4 is a top view of a die 30 implementing the filter circuit 10 a ofFIG. 3 in accordance with one embodiment of the present invention. InFIGS. 3 and 4, corresponding resonators are illustrated with samereference numerals. Connection points of FIG. 3 are illustrated asconnection pads in FIG. 4 and corresponding connection points andconnection pads are illustrated with same reference numerals. FIG. 5 isa cutaway side view of a portion of the die of FIG. 4 cut along lineA—A. For filter circuits filtering electronic signals in the gigahertzfrequency range, each of the resonators of the die 30 can have lateralsizes on the order of hundreds of microns or less and thicknesses in theorder of microns or less.

Referring to FIGS. 4 and 5, the die 30 includes shunt resonators 22, 24,26, and 28 fabricated on a substrate 32. In the illustrated embodimentof the present invention, series resonators 12 a, 14 a, and 16 a arefabricated above shunt resonators 22, 24, 26, respectively. For example,the shunt resonator 26 (the first acoustic resonator 26) is fabricatedon the substrate 32. The substrate 32 may include a cavity 34 under thefirst resonator 26. A second resonator, in this case series resonator 16a, is vertically separated above said first resonator 26. In theillustrated embodiment, the first and the second resonators 26 and 16 aare film bulk acoustic resonators (FBARS). The substrate can be, forexample, silicon substrate.

The first resonator 26 includes a bottom electrode 26 be and a topelectrode 26 te sandwiching a piezoelectric layer 26 pz. Likewise, thesecond resonator 16 a includes a bottom electrode 16 be and a topelectrode 16 te sandwiching a piezoelectric layer 16 pz. The electrodesof both the first and the second resonators 26 and 16 a are made fromconductive material such as, for example only, Molybdenum. Thepiezoelectric layer of both the first and the second resonators 26 and16 a are made from piezoelectric material such as, for example, AluminumNitride.

The second resonator 16 a is supported by standoffs 27 and is separatedand de-coupled from the first resonator 26 mostly by air. The distancebetween the first resonator 26 and the second resonator 16 can varywidely depending on implementation and can range, for example, from 0.1microns to 20 microns. In some embodiments, the distance between thefirst resonator 26 and the second resonator 16 can be maintained usingseparators 29. In FIG. 5, two separators 29 are illustrate; however, theseparators 29 are used to prevent the vertically separated resonators 26and 16 from touching each other. The separators 29 can be a short pillaror stub fabricated on the top electrode 26 te of the first resonator 26.The separators 29 are fabricated using similar process and material asthe standoffs 27. In the illustrated embodiment, top cross sectionalarea (the top cross section not illustrated) of the separators 29 arevery small compared to the area (partially illustrated in FIG. 4) of thetop electrode 26 te of the first resonator 26 and can be, for example,less than one percent of the area of the top electrode 26 te.

Because the acoustic resonators 26 and 16 are vertically andacoustically separated, little or no acoustic energy is coupled betweenthe first acoustic resonator 26 and the second acoustic resonator 16.

The standoffs 27 have height that is measured in the order ranging fromfractions of microns to tens or even hundreds of microns depending onimplementation. Lateral extents of the standoffs can range from 0.5microns to 100 microns defining a cross sectional area ranging from onemicron square to one millimeter square. The separation between theresonators can be but is not necessarily complete. For example, standoffcan be fabricated between the two resonators to separate the resonatorswhile the standoff itself can connect small portions of the separatedresonators.

The standoffs 27 can be fabricated anywhere under the second resonator16 a. In the illustrated embodiment, the standoffs 27 are fabricated onthe first resonator 26 and are connected to the bottom electrode 16 beof the second resonator 16 a. In particular, in the illustratedembodiment, the standoffs 27 are fabricated on the piezoelectric layer26 pz of the first resonator 26. In fact, the standoffs 27 can befabricated on other portions of the die 30. For instance, the standoffs27 can be fabricated on the substrate 32 or the top electrode 26 te ofthe first resonator 26. The standoffs 27 can be fabricated using the anysufficiently rigid material that is also suitable for integration withthe resonator fabrication process such as, for example, tungsten.

The die 30 of FIG. 3 requires an area (defined by the first and seconddimensional extents illustrated as X-axis extent 33 and Y-axis extent35) to implement the resonators. The some resonators of the die 30 arevertically separated above other resonators. For this reason, the die 30requires less area to implement all of its resonators compared to thedie 20 of FIG. 1.

FIG. 6 is a cutaway side view of the portion of the die 30 of FIG. 5 cutalong line A—A during its fabrication process. To fabricate the die 30including its vertically separated resonators, the first resonator 26 isfabricated on the substrate 32. In the illustrated embodiment, the firstresonator 26 is fabricated above the cavity 34. At this stage of thefabrication process, a cavity 34 is filled with some sacrificialmaterial such as, for example, phosphorus silicate glass (PSG). The PSGcan be the same material that is used as the sacrificial material forthe cavity 34. Then, a sacrificial layer 36 is fabricated surroundingthe first resonator 26. The standoffs 27 are also fabricated after thefabrication of the first resonator 26. The sacrificial layer 36 isplanarized by polishing using, for example, slurry.

Next, the second resonator 16 a is fabricated above the standoffs 27 andalso above the sacrificial layer 36. Finally, the sacrificial layer 36is removed leaving the second resonator 16 supported by the standoffs 27and suspended above the first resonator 26. To remove the PSGsacrificial layer, hydrofluoric acid can be used.

FIG. 7 is a schematic diagram of the filter circuit 10 of FIG. 1 redrawnto emphasize topology of resonators in accordance with anotherembodiment of the present invention. This redrawn filter circuit isdesignated using reference numeral 10 b. The filter circuit 10 b of FIG.7 includes identical component resonators and connection points as thefilter circuit 10 of FIG. 1. Further, operations of the filter circuit10 b of FIG. 7 are identical to operations of the filter circuit 10 ofFIG. 1. For this reason, same reference numerals are used for thecorresponding components in FIGS. 1 and 7 except as follows: in FIG. 7,to more clearly illustrate the vertically separating technique inaccordance with the illustrated embodiment of the present invention,resonators 12, 14, 16, 24, and 28 of FIG. 1 are referred to asresonators 12 b, 14 b, 16 b, 24 b, and 28 b in FIG. 7.

FIG. 8 is a cutaway side view a portion of a die 40 implementing thefilter circuit 10 b of FIG. 7 in accordance with the other embodiment ofthe present invention illustrating additional aspects of the presentinvention. In FIGS. 7 and 8, corresponding resonators are illustratedwith same reference numerals.

Referring to FIGS. 7 and 8, in the illustrated embodiment, the die 40includes shunt resonators 22 and 26 fabricated on a substrate 42. Seriesresonators 12 b, 14 b, and 16 b are fabricated vertically separatedabove the shunt resonators 22 and 26. Further, shunt resonators 24 b and26 b are fabricated vertically separated above the series resonators 12b, 14 b, and 16 b. As before the substrate 42 may include cavities 44under the resonators 22 and 26. Vertically separated above a firstresonator 26 is a second resonator 16 b. In the illustrated embodiment,the first and the second resonators 26 and 16 b are film bulk acousticresonators (FBARS). Here, a third resonator 28 b is fabricated above thesecond resonator 16 b. Because of additional vertically separating ofthe resonators, the die 40 of FIG. 8 requires even less space then thedie 30 of FIG. 4.

FIG. 8 illustrates additional aspects of the present invention. In FIG.8, standoffs for supporting vertically separated resonators aredesignated using reference numeral 41 followed by a letter beginningwith letter “a.” Not all standoffs are thus designated. In theillustrated embodiment, one of the standoffs, standoff 41 a, isfabricated on the substrate 42 while others such as standoff 41 b arefabricated on top electrodes of lower resonators. In fact, the standoff41 b is situated between top electrode 26 te of the first resonator 26and bottom electrode 16 be of the second resonator 16 b and mechanicallyconnects the top electrode 26 te of the first resonator 26 to the bottomelectrode 16 be of the second resonator 16 b.

If the standoff 41 b is made from electrically conductive material suchas, for example, tungsten or even Molybdenum, the same material as theelectrodes, then the top electrode 26 te of the first resonator 26 andthe bottom electrode 16 be of the second resonator 16 b are electricallyconnected. Alternatively, if the standoff 41 b is made from electricallyinsulating material, then the top electrode 26 te of the first resonator26 and the bottom electrode 16 be of the second resonator 16 b areelectrically separated from each other and may have different electricalpotential relative to each other. In this case, a capacitive potentialis created between the top electrode 26 te of the first resonator 26 andthe bottom electrode 16 be of the second resonator 16 b. For electricalseparation, the standoffs 41 b can be made from electrically conductingmaterial such as Tungsten or Molybdenum, or insulating orsemi-insulating materials such as silicon nitride or polysilicon.

From the foregoing, it will be apparent that the present invention isnovel and offers advantages over the current art. Although specificembodiments of the invention are described and illustrated above, theinvention is not to be limited to the specific forms or arrangements ofparts so described and illustrated. For example, differingconfigurations, sizes, or materials may be used but still fall withinthe scope of the present invention. The invention is limited by theclaims that follow.

1. An apparatus comprising: a first film bulk acoustic resonator (FBAR)on a substrate; a second FBAR above said first FBAR, said second FBARbeing vertically separated from said first FBAR such that little or noacoustic energy is coupled between said first FBAR and said second FBAR;at least one standoff fabricated directly on said first FBAR, saidstandoff supporting said second FBAR; and wherein said standoffelectrically connects said first acoustic resonator and said secondacoustic resonator.
 2. The apparatus recited in claim 1 wherein saidfirst FBAR includes a bottom electrode and a top electrode sandwiching apiezoelectric layer; and wherein said second FBAR includes a bottomelectrode and a top electrode sandwiching a piezoelectric layer.
 3. Theapparatus recited in claim 1 wherein the standoff is connected to abottom electrode of said second FBAR.
 4. The apparatus recited in claim1 wherein said standoff is selected from the group consisting oftungsten and molybdenum.
 5. The apparatus recited in claim 1 whereinsaid first FBAR is de-coupled from said second FBAR by air.
 6. Theapparatus recited in claim 1 wherein a distance between said first FBARand said second FBAR is within a range from 0.1 microns to 20 microns.7. The apparatus recited in claim 1 further comprising a third FBARvertically separated above said second FBAR.
 8. The apparatus recited inclaim 1, wherein said first FBAR is suspended over a cavity in saidsubstrate.
 9. The apparatus recited in claim 8, further comprising atleast one separator on said first FBAR to prevent said first FBAR andsaid second FBAR from touching each other under vibration.
 10. Anapparatus comprising: a first film bulk acoustic resonator (FBAR) on asubstrate; a second FBAR above said first FBAR, said second FBAR beingvertically separated from said first FBAR such that little or noacoustic energy is coupled between said first FBAR and said second FBAR;at least one standoff fabricated directly on said first FBAR, saidstandoff supporting said second FBAR; and wherein a top electrode ofsaid first FBAR and a bottom electrode of said second FBAR are atdifferent electrical potentials relative to each other thereby creatinga capacitive potential between the top electrode of said first FBAR andthe bottom electrode of said second FBAR.
 11. The apparatus recited inclaim 10 wherein said first FBAR includes a bottom electrode and a topelectrode sandwiching a piezoelectric layer; and wherein said secondFBAR includes a bottom electrode and a top electrode sandwiching apiezoelectric layer.
 12. The apparatus recited in claim 10 wherein thestandoff is connected to a bottom electrode of said second FBAR.
 13. Theapparatus recited in claim 10 wherein said standoff is selected from thegroup consisting of silicon nitride and polysilicon.
 14. The apparatusrecited in claim 10 wherein said first FBAR is de-coupled from saidsecond FBAR by air.
 15. The apparatus recited in claim 10 wherein adistance between said first FBAR and said second FBAR is within a rangefrom 0.1 microns to 20 microns.
 16. The apparatus recited in claim 10further comprising a third FBAR vertically separated above said secondFBAR.
 17. The apparatus recited in claim 10, wherein said first FBAR issuspended over a cavity in said substrate.
 18. The apparatus recited inclaim 17, further comprising at least one separator on said first FBARto prevent said first FBAR and said second FBAR from touching each otherunder vibration.